12 - Alkanes Flashcards
Are alkanes saturated or unsaturated?
Saturated
Physical Properties of alkanes
- polarity
- boiling points
- solubility
- reactivity
Polarity
Alkanes are almost non polar because the electronegativities of carbon and hydrogen are similar
The intermolecular forces between their molecules are weak van dear Waals forces
The larger the molecule, the larger the vdW force
Solubility of alkanes
Insoluble in water
Water molecules are held together by hydrogen bonds which are much stronger than the vdW forces that act between alkane molecules,
Boiling points of alkanes
As the length of the alkane chain increases, so does the boiling point. This is because the intermolecular forces increase meaning more energy is required to break the bonds.
Reactivity of alkanes
Alkanes are relatively unreal it. They have strong carbon-carbon and carbon-hydrogen bonds
They don’t react with: acids, bases, oxidising agents or reducing agents.
However, they burn and react with halogens
Products of complete combustion
Carbon dioxide + Water
Products of incomplete combustion
Carbon/Carbon monoxide + water
When to know if carbon monoxide or carbon is produces
Carbon monoxide: a poisonous gas is produces
Carbon: black product (soot)
What are alkanes used as
Fuels
Why are alkanes good fuels
Combustion reactions give out heat and have negative enthalpy changes. Which means lots of heat is given out basically so it is a good fuel
Different pollutants produced as a results of burning alkanes
Carbon monoxide
Nitrogen oxides
Sulfur dioxide
Carbon particulates
Unburnt hydrocarbons
Carbon dioxide
Water vapour
Why is carbon monoxide a pollutant
Poisonous gas
Why are nitrogen oxides a pollutant
Oxides react with water vapour and oxygen in the air to form nitric acid —> acid rain
How are nitrogen oxides produced?
Under extreme heat and pressure
N2 + O2 -> 2NO
How is Sulfur dioxide produced?
Produced from Sulfur containing impurities present in crude oil
Negativity of Sulfur dioxide
Combines with water vapour and oxygen in the air to form Sulfuric acid —> acid rain
Negatives of carbon particles
Very small, easy to enter the body
Negatives of unburnt hydrocarbons
Easy for them to enter the atmosphere and they are significant greenhouse gases.
They contribute to photochemical smog which can cause a variety of health problems
Negatives of CO2
Greenhouse gas -> climate change etc
Negative of water vapour
Greenhouse gas
Sulfur dioxide gas ‘nickname’
Flue gases
Equation for how sulfuric acid is produced
SO2 + 0.5O2 + H2O —> H2SO4
How can calcium oxide remove flue gas?
Calcium oxide (lime) and water are sprayed into the flue gas which forms calcium sulphite
This is further oxidised to calcium sulphate aka gypsum
Equation for calcium sulphide flue gas desulfurisation
CaO + 2H20 + SO2 + 0.5O2 -> CaSO4.2H20
Equation for the calcium carbonate (limestone) flue gas desulfurisation
CaCo3 + 0.5O2 + SO2 —> CaSO4 + CO2
What is a catalytic converter?
A honeycomb,b made of a ceramic material coated with ceramic and rhodium metals
They reduce the output of carbon monoxide, nitrogen oxides and unburnt hydrocarbons in the exhaust gas mixture
Equation for how a catalytic converter removes carbon monoxide and nitrogen oxide
2CO + NO —> N2 +2CO2
Equation for how a catalytic converter removes hydrocarbons and nitrogen oxide
Hydrocarbons + Nitrogen oxide —> Nitrogen + Carbon Dioxide + Water
Eg C9H18 + 25NO —> 12.5N2 + 8CO2 + 9H20
What is a free radical?
A free radical is any species with an unpaired electron
What is needed for free radical substitution to occur
UV light
Initiation step with free radical substitution
First, you need to break the covalent bond between the halogens
Eg: Cl—Cl—> 2Cl•
Why does the Cl bond break?
The chlorine molecule absorbs the energy of a single quantum of UV light. The energy of one quantum of UV light is greater than the Cl—Cl bond energy, so the bond will break.
Propagation step 1, what happens?
The chlorine free radical takes a hydrogen atom from the alkane to form hydrogen (halogen)
CH4 + Cl• —> CH3• + HCl
Propagation step 2, what happens?
The methyl free radical is now very reactive and reacts with a chlorine e molecule. This produces another chlorine free radical and a molecule of chloromethane
•CH3 + Cl2 —> CH3Cl + Cl•
What is the effect of propagation?
To produce hydrogen chloride, chloromethane and a new Cl• free radical. This is ready to react with more methane and repeat the 2 steps.
What happens in termination
The free radicals are removed
This can happen in 3 ways:
Cl. + Cl. —> Cl2
.CH3 + .CH3 —> C2H6
Cl. + .CH3 —> CH3Cl
What substances are destroying the ozone layer?
Chlorofluorocarbons (CFCs)
Why is the ozone layer important?
It protects the Earth from the harmful exposure to too many UV rays.
How are chlorine free radicals formed?
Chlorine free radicals are formed from CFCs because the C—Cl bond breaks homolytically in the presence of UV radiation to produce chlorine free radicals, Cl. These then attack the ozone molecules
Equation for how Cl free radicals attack the ozone:
Cl. + O3 —> ClO. + O2
ClO. + O2 —> 2O2 + Cl.
The chlorine free radical isn’t destroyed, it acts as a catalyst in the breakdown of ozone to oxygen
2O3 —> 3O2
